This invention relates to a device provided with a nitride semiconductor InxAlyGa1xe2x88x92xxe2x88x92yN (0=x, 0=y, x+y=1) including light emitting devices such as LED (light emitting diode), LD (laser diode) and SLD (super luminescent diode), solar cells, light receiving devices such as optical sensors and electronic devices such as transistors and power devices.
Nitride semiconductors have been recently produced as materials used for a high bright blue LED and a pure green LED, a full color LED display and a traffic signal LED. Such LEDs are provided with an active layer of SQW (Single Quantum Well) or MQW (Multi Quantum Well) where the well layer is made of InGaN and positioned between a p-type nitride layer and an n-type nitride layer to form a DH (Double Hetero) structure. The wavelength of the blue or green light emitting from the active layer depends on a ratio of In in the InGaN well layer.
The inventors have first realized laser emitting by using the above nitride materials and reported it in Jpn. J. Appl. Phys. 35(1996)L74 and Jpn. J. Appl. Phys. 35(1996)L217. The laser device comprises the DH structure where the active layer is MQW having InGaN well layers and showed the following data:
Threshold current: 610 mA;
Threshold current density: 8.7 kA/m2;
Wavelength: 410 nm
(pulse width 2 xcexcm and pulse cycle 2 ms) p The inventors have further improved the laser device and reported it in Appl. Phys. Lett. 69(1996)1477. The laser device comprises a ridge strip structure formed on a part of p-type nitride semiconductor and showed the following data.
Threshold current: 187 mA;
Threshold current density: 3 kA/m2;
Wavelength: 410 nm
(pulse width 2 xcexcm, pulse cycle 2 ms and duty ratio: 0.1%)
The inventors have first succeeded in CW (Continuous-Wave) Oscillation or Operation at room temperature and reported it in Gijutsu-Sokuho of Nikkei Electronics issued on Dec. 2, 1996, Appl. Phys. Lett. 69(1996) and Appl. Phys. Lett. 69(1996)4056.
The laser diode showed a lifetime of 27 hours at 20xc2x0 C. under the threshold current density of 3.6 ka/cm2 the threshold voltage of 5.5 V and the output of 1.5 mW.
On the other hand, the blue and green LED of nitrides showed a forward current (If) of 20 mA and a forward voltage (Vf) of 3.4 to 3.6 V which are higher by 2 V or more than those of red LEDs made of GaAlAs semiconductors. Therefore, further decrease of Vf in the blue and green LED was required. Additionally, there was required an effective LD which can decrease the threshold current and voltage to get a longer lifetime of CW operation at room temperature, because the conventional LD still had a higher threshold current and voltage.
The inventors have gotten the idea that technology of decreasing the threshold in LDs was applicable to LEDs in order to decrease the Vf. Therefore, a first object of the present invention is to decrease the threshold current and voltage of nitride semiconductor LDs and realize a longer lifetime of CW operation.
In the specification, it should be understood that the general formulae: InxGa1xe2x88x92xN and AlyGa1xe2x88x92yN show chemical atoms which compose of nitride layers and therefore, even if different layers are represented by the same formula, the different layers do not necessarily have the same composition, that is, the same x or y does not mean the same ratio.
According to a first aspect of the present invention, there is provided a nitride semiconductor device comprising a p-type region comprising one or more p-conductivity semiconductor layers of nitride, a n-type region comprising one or more n-conductivity semiconductor layers of nitride and an active layer of a nitride semiconductor which is positioned between said p type region and said n type region, at least one layer of said p type region being a super lattice layer comprising first thin layers of nitride and second thin layers of nitride, said first layers having different composition from those of said second layers and the first and second thin layers being laminated alternately.
The super lattice structure can make the nitride layers improved in crystallinity and then make the nitride layers decreased in resistivity, resulting in smaller resistance of the p-type region and higher power efficiency of the device.
In the present invention, the p-type region means a region comprising one or more nitride semiconductor layers between an active layer and a p-electrode while the n-type region means a region comprising one or more nitride semiconductor layers between the active layer and an n-electrode.
According to a second aspect of the present invention, there is provided a nitride semiconductor device having an active layer made of a nitride semiconductor between the n-type region of one or more nitride semiconductor layers and the p-type region of one or more nitride semiconductor layers, at least one semiconductor layer in the p-type region or the n-type region is a super lattice layer made by laminating first layers and second layers which are made of nitride semiconductor, respectively, and have different constitutions from each other.
The super lattice structure can make the nitride layers improved in crystallinity and then make the nitride layers decreased in resistivity, resulting in smaller resistance of the n-type region and higher power efficiency of the device.
In a preferred embodiment of the first and second nitride semiconductor devices, the super lattice layer is made by laminating first layers which is made of a nitride semiconductor and has a thickness of not more than 100 angstroms and second layers which is made of a nitride semiconductor having different constitutions from the first layer and has a thickness of not more than 100 angstroms.
In order to keep or confine carriers in the active layer, at least one of the first and second layers is preferably made of a nitride semiconductor containing Al, especially AlYGa1xe2x88x92YN (0 less than Yxe2x89xa61).
In a second preferred embodiment of the first and second nitride semiconductor devices, for the super lattice, the first layer is preferably made of a nitride semiconductor represented by the formula InXGa1xe2x88x92XN (0xe2x89xa6X xe2x89xa61) and the second layer is preferably made of a nitride semiconductor represented by the formula AlYGa1xe2x88x92YN (0 less than Yxe2x89xa61, X=Yxe2x89xa00). According to the second embodiment, all the nitride layers have a good crystallinity, which results in improving output of the nitride semiconductor device (improvement of power efficiency). In LED or LD devices, the forward voltage (hereinafter referred to Vf) and also the threshold current and voltage can be lowered. In order to form a nitride layer having better crystallinity in the first and second semiconductor device, it is further recommendable that first layers of the super lattice structure are made of a nitride semiconductor represented by the formula InXGa1xe2x88x92XN (0xe2x89xa6X less than 1) and said second layer is made of a nitride semiconductor represented by the formula AlYGa1xe2x88x92YN (0 less than Y less than 1).
In the above first and second semiconductor devices, it is preferable that the first layer and the second layer are made of a nitride semiconductor and have a thickness of not more than 70, especially 40 angstroms, respectively, while said first layer and said second layer have a thickness of not less than 10, especially 5 angstroms, respectively. The thickness within the above range makes it easy to form AlxGa1xe2x88x92yN (0 less than Yxe2x89xa61), which layer is otherwise difficult to be formed with a good crystallinity. Especially, in case that the super lattice layer can be made as at least one layer of the p-type region between the p-electrode and the active layer and also as at least one layer of the n-type semiconductor region between the n-contact layer for current charging and the active layer, it is recommendable to get better effect that thickness of the first and second layer should be set within the above range.
In the above embodiment of the first and second nitride semiconductor devices, the p-type region is preferably provided with a p-side contact layer having a thickness of not more than 500 angstroms, on which the p-electrode is to be formed. More preferably, the p-side contact layer has a thickness of not more than 300 angstroms and not less than 10 angstroms.
In the second nitride semiconductor device of the present invention, wherein the p-type region is provided with a p-side contact layer on which the p-electrode is to be formed, the super lattice layer is preferably formed between the active layer and the p-side contact layer.
Further, in the second nitride semiconductor device of the present invention, the n-type region may comprise a second buffer layer made of a nitride semiconductor which has a thickness of not less than 0.1 xcexcm via a first buffer layer on the substrate, an n-side contact layer made of a nitride semiconductor doped with an n-type impurity on said second buffer layer, and an n-electrode being formed on the n-side contact layer. This construction makes the n-side contact layer have higher carrier concentration and good crystallinity. In order to make the n-side contact layer have much better crystallinity, it is preferable that the concentration of the impurity in second buffer layer is lower than that in said n-side contact layer. Further, it is preferable that at least one of the first and second buffer layers is a super lattice layer made by laminating nitride semiconductor layers of different constitutions with a thickness of not more than 100 angstroms in order to make a nitride layer formed on the buffer layer and have a good crystallinity.
In the second nitride semiconductor device, wherein the n-type region has a n-side contact layer on which a n-electrode is to be formed, the super lattice layer is preferably formed between the active layer and the n-side contact layer. In the LD device, the layer formed between the active layer and the n-side contact layer or between the active layer and the p-side contact layer may be a cladding layer acting as a carrier keeping layer or a light guide layer, which is preferably made of the super lattice structure. Thereby, the super lattice structure can remarkably decrease the threshold current and voltage. Especially, if the p-cladding layer between the active layer and the p-side contact layer, the p-cladding layer of the super lattice structure is advantageous to lower the threshold current and voltage. In the second nitride semiconductor device of the present invention, it is preferable that at least one of said first layer and said second layer is doped with an impurity which makes the conductivity of the layer n-type or p-type and the impurity concentration doped to the first layer and the second layer to make the conductivity of the layers n-type or p-type, are different from each other. The impurity for making the conductivity of the layer includes n-impurities belonging to IV-A, IV-B, VI-A and VI-B groups and p-impurities belonging to I-A, I-B, II-A, II-B groups (hereinafter referred to n-impurity and p-impurity).
In the second nitride semiconductor device of the present invention, the super lattice layer may be formed as the n-side contact layer on which the n-electrode is to be formed, whereby the resistance of n-side contact layer can be lowered, resulting in further decreasing of the threshold current and voltage in LD devices.
In the LD devices provided with the first or second nitride semiconductor device of the present invention, if the laser device has a super lattice layer in the p-type region, a ridge portion may be formed on the supper lattice layer and on the layer located over said super lattice layer in a manner that the longitudinal direction of the ridge portion coincides with the direction of resonance and the ridge has a predetermined width.
In a preferred first laser diode of the present invention, which comprises an active layer in which laser is emitted between the n-type region including a n-side cladding layer and the p-type region including a p-side cladding layer, the n-side cladding layer may be a super lattice layer made by laminating first layers made of a nitride semiconductor having a thickness of not more than 100 angstroms and second layers made of a nitride semiconductor of a different constitution from the first layer and having a thickness of not more than 100 angstroms, and said p-side cladding layer may be a super lattice layer made by laminating a third layer made of a nitride semiconductor having a thickness of not more than 100 angstroms and a fourth layer made of a nitride semiconductor of a different constitution from the third layer and having a thickness of not more than 100 angstroms. Due to this, during laser emission the threshold current and voltage can be lowered. In this case, the ridge portion may be formed on said p-side cladding layer and on the layer located over said p-side cladding layer in a manner that the longitudinal direction of the ridge coincidences with the direction of resonance and the ridge has a desired width.
According to a third aspect of the present invention, there is provided a third nitride semiconductor device which comprises an active layer made of a nitride semiconductor between a n-type region of one or more nitride semiconductor layers and a p-type region of one or more nitride semiconductor layers, wherein at least one nitride semiconductor layer in the n-type region is a n-side super lattice made by laminating first and second nitride semiconductor layers which have different constitutions and different concentrations of a n-type impurity from each other. Due to this construction, the nitride semiconductor layer made of the super lattice structure makes the electrical resistance thereof smaller and thus the total resistance of the n-type region can be smaller.
According to a fourth aspect of the present invention, there is provided a nitride semiconductor device comprising an active layer made of a nitride semiconductor between the n-type region of one or more nitride semiconductor layers and the p-type region of one or more nitride semiconductor layers, characterized in that at least one nitride semiconductor layer in the p-type region is a p-side super lattice made by laminating third and fourth nitride semiconductor layers which have different constitutions and different concentrations of a p-type impurity from each other. The super lattice structure can make the nitride semiconductor layer comprising the super lattice structure have a lower resistance and then total resistance of the p-type region can be decreased.
Please note that the first and second and the third and fourth of layers does not mean the laminating order in the specification.
According to a fifth aspect of the present invention, there is provided a nitride semiconductor device comprising an active layer made of a nitride semiconductor between the n-type region of one or more nitride semiconductor layers and the p-type semiconductor region of one or more nitride semiconductor layers, characterized in that at least one nitride semiconductor layer in the n-type region is a n-side super lattice made by laminating the first and second nitride semiconductor layers which have different constitutions and different concentrations of a n-type impurity from each other, and at least one nitride semiconductor layer in p-type region is a p-side super lattice made by laminating the third and fourth nitride semiconductor layers which have different constitutions and different concentrations of a p-type impurity from each other. The super lattice structure can make the resistance of the nitride semiconductor comprising super lattice structure smaller and thus total resistance of the p-type region can be decreased.
In a case that the third and fifth semiconductor devices are devices of optoelectronics such as light emitting devices and light receiving devices, the n-side super lattice layer may be formed as at least one of the group consisting of a buffer layer formed on the substrate, a n-side contact layer for n-electrode, n-side cladding layer for confining or keeping carriers and n-side light guide layer for guiding emission from the active layer. On the other hand, in the fourth and fifth semiconductor device, the p-side super lattice layer may be formed as at least one selected from the group consisting of the p-side contact layer, the p-side cladding layer for confining carriers and the p-side wave guide layer for guiding emission from the active layer.
In the third and fifth semiconductor devices of the present invention, for the n-side super lattice layer, the first nitride semiconductor layer having a higher band gap may have a larger or smaller concentration of the n-type impurity than the second nitride semiconductor layer having a lower band gap. The larger impurity concentration of the first layer than that of the second layer makes carrier generate in the first layer having a higher band gap and then the carrier injected into the second layer having a lower band gap to move the carrier through the second layer having a smaller impurity concentration and a larger mobility. Therefore, this construction makes the n-side super lattice layer decreased in electrical resistance.
In a case that the impurity concentration of the first layer is relatively larger than that of the second layer, the first layer of the super lattice layer in the first semiconductor device may decrease the n or p-impurity concentration at a part close to the second layer comparing with that at a part remote from the second layer, which prevents the carrier moving through the second layer from scattering by the impurity at the part close to the second layer, resulting in increase of mobility of the second layer and thus lowering of the resistance of the super lattice layer.
In the embodiment of the third and fifth nitride semiconductor devices, if the n-impurity concentration in the first layer having a higher band gap becomes larger, it is preferable that the n-impurity concentration in the first layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3 and the n-impurity concentration in the second layer is smaller than that of the first layer and not more than 1xc3x971019/cm3. The n-impurity concentration in the second layer having a smaller band gap is preferably not more than 1xc3x971018/cm3, more preferably not more than 1xc3x971017/cm3. From the aspect of increasing the mobility of the second layer, a smaller n-impurity concentration is better and an undoped layer or intentionally not doped layer is most preferable.
If the impurity concentration of the first layer is smaller than that of the second layer, it is preferable that the n-impurity concentration of the second layer is smaller at a part close to the first layer than that at a part remote from the first layer. For example, it is preferable that the n-impurity concentration in the first layer is not more than 1xc3x971019/cm3 and the n-impurity in the second layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3. The n-impurity concentration in the first layer having a smaller band gap is preferably not more than 1xc3x971018/cm3, more preferably not more than 1xc3x971017/cm3. The most preferable first layer is an undoped layer or intentionally not doped layer.
In order to form an n-side super lattice layer having a good crystallinity in the third and fifth semiconductor device, the first nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1) capable of forming a relatively higher band gap layer having a good crystallinity and the second nitride semiconductor layer may be made of InXGa1xe2x88x92XN (0xe2x89xa6X less than 1) capable of forming a relatively smaller band gap layer having a good crystallinity. The best second layer of the super lattice layer in the third and fifth semiconductor devices, is a GaN layer. This construction is advantageous in manufacturing the super lattice layer because the same atmosphere can be used to form the first layer (AlYGa1xe2x88x92YN) and the second layer (GaN).
In the third and fifth nitride semiconductor devices, the first nitride semiconductor layer may be made of AlXGa1xe2x88x92XN (0 less than X less than 1) and the second nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1, X greater than Y). In this case, further, the first nitride semiconductor layer or said second nitride semiconductor layer is preferably not doped with a n-type impurity.
In the fourth and fifth semiconductor devices of the present invention, for the p-side super lattice layer, the third nitride semiconductor layer having a higher band gap may have a larger or smaller concentration of the p-type impurity than that of the fourth nitride semiconductor layer having a smaller band gap. The larger impurity concentration of the third layer than that of the fourth layer makes carriers generate in the third layer having a higher band gap, and the carriers injected into the fourth layer having a smaller band gap to move the injected carriers through the fourth layer having a smaller impurity concentration and a larger mobility, resulting in decreasing the super lattice resistance.
Further, in the fourth and fifth semiconductor devices of the present invention, it is preferable that a part of the third nitride semiconductor layer which is close to the fourth nitride semiconductor layer has a lower concentration of the p-type impurity than a part remote or farther from the fourth nitride semiconductor layer, which prevents the carrier moving through the fourth layer from scattering by the impurity at the part close to the fourth layer, resulting in increase of mobility of the fourth layer and thus further lowering of the resistance of the super lattice layer.
In the embodiment of the fourth and fifth nitride semiconductor devices, if the n-impurity concentration in the third layer becomes larger than that in the fourth layer, it is preferable that the n-impurity concentration in the third layer having a larger band gap ranges between 1xc3x971018/cm3 and 1xc3x971021/cm3 and the p-impurity concentration in the fourth layer is smaller than that of the third layer and not more than 1xc3x971020/cm3. The p-impurity concentration in the fourth layer having a smaller band gap is preferably not more than 1xc3x971019/cm3, more preferably not more than 1xc3x971018/cm3. From the aspect of increasing the mobility of the second layer, a smaller n-impurity concentration is better and an undoped layer or intentionally not doped layer is most preferable.
In the fourth and fifth nitride semiconductor, if the impurity concentration of the third layer is smaller than that of the fourth layer, it is preferable that the p-impurity concentration of the fourth layer is smaller at a part close to the third layer than that at a part remote from the third layer. For example, it is preferable that the p-impurity concentration in the first layer is not more than 1xc3x971020/cm3 and the n-impurity in the second layer ranges between 1xc3x971018/cm3 and 1xc3x971021/cm3. The n-impurity concentration in the third layer having a smaller band gap is preferably not more than 1xc3x971019/cm3, more preferably not more than 1xc3x971018/cm3. The most preferable first layer is an undoped layer or intentionally not doped layer.
In order to form a super lattice layer having a good crystallinity in the fourth and fifth semiconductor device, the third nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1) capable of forming a relatively higher band gap layer having a good crystallinity and the fourth nitride semiconductor layer may be made of InXGa1xe2x88x92XN (0xe2x89xa6X less than 1). The best fourth layer of the super lattice layer in the third and fifth semiconductor devices, is a GaN layer. This construction is advantageous in manufacturing the super lattice layer because the same atmosphere can be used to form the third layer (AlYGa1xe2x88x92YN) and the fourth layer (GaN).
In the fourth and fifth nitride semiconductor devices, the third nitride semiconductor layer may be made of AlXGa1xe2x88x92XN (0 less than X less than 1) and the fourth nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1, X greater than Y). In this case, further, the third nitride semiconductor layer or the fourth second nitride semiconductor layer is preferably not doped with a n-type impurity.
In the fifth nitride semiconductor, for the n-side super lattice layer, the first nitride semiconductor layer may be provided with a higher band gap energy and a larger concentration of the n-type impurity than the second nitride semiconductor layer, and for the p-side super lattice layer, the third nitride semiconductor layer may be provided with a higher band gap energy and a larger concentration of the p-type impurity than the fourth nitride semiconductor layer. In this case, it is preferable that the concentration of the n-type impurity in the first nitride semiconductor layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3 and the concentration of the n-type impurity in the second nitride semiconductor layer is not more than 1xc3x971019/cm3, and the concentration of the p-type impurity in the third nitride semiconductor layer ranges between 1xc3x971018/cm3 and 1xc3x971021/cm3 and the concentration of the p-type impurity in the fourth nitride semiconductor layer is not more than 1xc3x971020/cm3.
Further, in the fifth nitride semiconductor device, for the n-side super lattice layer, the first nitride semiconductor layer may be provided with a higher band gap energy and a larger concentration of the n-type impurity than said second nitride semiconductor layer, and for the p-side super lattice layer, the third nitride semiconductor layer may be provided with a higher band gap energy and a smaller concentration of the p-type impurity than the fourth nitride semiconductor layer. In this case, it is preferable that the concentration of the n-type impurity in the first nitride semiconductor layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3 and the concentration of the n-type impurity in the second nitride semiconductor layer is not more than 1xc3x971019/cm3, and the concentration of the p-type impurity in the third nitride semiconductor layer is not more than 1xc3x971020/cm3 and the concentration of the p-type impurity in the fourth nitride semiconductor layer ranges between 1xc3x971018/cm3 and 1xc3x971021/cm3.
Furthermore, in the fifth nitride semiconductor device, or the n-side super lattice layer, the first nitride semiconductor layer may be designed to have a higher band gap energy and a smaller concentration of the n-type impurity than the second nitride semiconductor layer, and for the p-side super lattice layer, the third nitride semiconductor layer may be designed to have a higher band gap energy and a larger concentration of the p-type impurity than the fourth nitride semiconductor layer. In this case, it is preferable that the concentration of the n-type impurity in the first nitride semiconductor layer is not more than 1xc3x971019/cm3 and the concentration of the n-type impurity in the second nitride semiconductor layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3, and the concentration of the p-type impurity in the third nitride semiconductor layer ranges between 1xc3x971018/cm3 and 1xc3x971021/cm3 and the concentration of the p-type impurity in the fourth nitride semiconductor layer is not more than 1xc3x971020/cm3.
Further, in the fifth nitride semiconductor device, for the n-side super lattice layer, the first nitride semiconductor layer may be designed to have a higher band gap energy and a smaller concentration of the n-type impurity than the second nitride semiconductor layer, and for the p-side super lattice layer, the third nitride semiconductor layer may be designed to have a higher band gap energy and a smaller concentration of the p-type impurity than the fourth nitride semiconductor layer. In this case, it is preferable that the concentration of the n-type impurity in the first nitride semiconductor layer is not more than 1xc3x971019/cm3 and the concentration of the n-type impurity in the second nitride semiconductor layer ranges between 1xc3x971017/cm3 and 1xc3x971020/cm3, and the concentration of the p-type impurity in the third nitride semiconductor layer is not more than 1xc3x971020/cm3 and the concentration of the p-type impurity in the fourth nitride semiconductor layer ranges between 1xc3x971018/cm3xcx9c1xc3x971021/cm3.
Furthermore, in the fifth nitride semiconductor device, for the n-side super lattice layer, the first nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1) and the second nitride semiconductor layer may be made of InXGaxe2x88x92XN (0xe2x89xa6X less than 1), and for the p-side super lattice layer, the third nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1) and the fourth nitride semiconductor layer maybe made of InXGa1xe2x88x92XN (0xe2x89xa6X less than 1). In this case, it is preferable that the second and fourth nitride semiconductor layers are made of GaN, respectively.
Further, in the fifth nitride semiconductor device, for the n-side super lattice layer, the first nitride semiconductor layer may be made of AlXGa1xe2x88x92XN (0 less than X less than 1) and the second nitride semiconductor layer is made of AlYGa1xe2x88x92YN (0 less than Y less than 1, X greater than Y), and for the p-side super lattice layer, the third nitride semiconductor layer may be made of AlXGa1xe2x88x92XN (0 less than X less than 1) and the fourth nitride semiconductor layer may be made of AlYGa1xe2x88x92YN (0 less than Y less than 1, X greater than Y).
Furthermore, in the fifth nitride semiconductor device, it is preferable that the first nitride semiconductor layer or the second nitride semiconductor layer is an undoped layer to which a n-type impurity is not doped. It is also preferable that the third nitride semiconductor layer or the fourth nitride semiconductor layer is an undoped layer which is not doped with a p-type impurity.
In the third, fourth and fifth nitride semiconductor device, the active layer preferably includes a InGaN layer. The InGaN layer in the active layer is preferably in a form of a quantum well layer. The active layer may be SQW or MQW.
According to the present invention, there is provided a second nitride semiconductor LD device comprising an active layer between a p-side cladding layer and a n-side cladding layer, and at least one of the p-side and the n-side cladding layers is the n-side super lattice layer or the p-side super lattice layer respectively. The LD device can operate at a lower threshold current. In the second LD device, it is preferable that an optical wave guide layer made of a nitride semiconductor containing In or GaN which has an impurity concentration of not more than 1xc3x971019/cm3, the optical wave guide layer being formed at least either between the p-side cladding layer and the active layer or between the p-side cladding layer and the active layer. In this case, the wave guide can prevent the emission generated from disappearing due to a low absorption rate of the optical wave guide, which causes a LD device capable of waving at a low gain. In this case, in order to further decrease the light absorption rate, it is more preferable that the impurity concentration of the wave guide layer is not more than 1xc3x971018/cm3, especially not more than 1xc3x971017/cm3. The most preferable layer is an undoped one. The optical wave guide layer may be made of the super lattice structure.
Furthermore, it is recommendable that there is provided with a cap layer made of a nitride semiconductor between the optical wave guide layer and the active layer. It is preferable that the cap layer having a higher band gap energy than the well layer in the active layer and also the optical wave guide layer and having a thickness of not more than 0.1 xcexcm is formed between said optical wave guide layer and said active layer. It is more preferable that the cap layer contains an impurity of not less than 1xc3x971018/cm3. The cap layer can make a leak current lowered because of a higher band gap. It is effective that the optical wave guide layer and cap layer are formed in the p-type region or the semiconductor region of p-conductivity side.
The third to the fifth nitride semiconductor devices of the present invention may be preferably formed on a nitride semiconductor substrate. The nitride semiconductor substrate can be prepared by a method of growing a nitride semiconductor layer on an auxiliary substrate made of a material other than nitride semiconductor, forming a protective film on the grown nitride semiconductor layer so as to expose partially the surface thereof, thereafter growing a nitride semiconductor layer to cover the protective film from the exposed nitride semiconductor layer. The nitride semiconductor substrate can make it better the crystallinity of every layers in the third to the fifth nitride semiconductor device. In this case, the auxiliary substrate and the protective film can be removed from the nitride semiconductor substrate before or after the device layers are formed on the nitride semiconductor substrate. The cap layer had better be formed in the p-type region.
In a preferred embodiment of the LD device according to the present invention, wherein p-side cladding layer is a super lattice layer, it is preferable that a ridge portion is formed on the p-side cladding layer and on the layer located over the p-side cladding layer in a manner that the longitudinal direction of the ridge portion coincides with the direction of resonance and the ridge has a predetermined width.
According to a sixth aspect of the present invention, there is provided a nitride semiconductor light emitting device comprising an active layer including a first nitride semiconductor layer containing In between a n-side cladding layer and a p-side cladding layer, characterized in that the n-side cladding layer is a super lattice layer comprising a second nitride semiconductor layer containing Al and has a total thickness of not less than 0.5 xcexcm and an average composition of Al in said n-side cladding layer is set in a way that the product of said average Al composition in % contained in said n-side cladding layer multiplied by the thickness in xcexcm of said n-side cladding layer is not less than 4.4. This causes the optical confinement effect by the n-side cladding layer improved, resulting in a long lifetime and a high responsibility of the LD device due to a lower wave oscillation threshold.
In an embodiment of the LD device formed on the substrate, wherein the n-side cladding layer is usually formed at a part close to the substrate in the n-type region, if the confinement effect of the light is not sufficient, the light leaked through the n-side cladding layer is reflected by the substrate, resulting in disturbing shapes of far and near field pattern such as observation of multi-spots of laser beam. However, the n-side cladding layer in the sixth nitride semiconductor device, makes the light confinement effect improved, which prevent the near and far field patterns from being disturbed, that is, which can make a single laser spot.
In a preferred embodiment of the sixth nitride semiconductor device of the present invention, the n-side cladding layer has a thickness of not less than 0.8 xcexcm and an average Al composition of not less than 5.5%. In a more preferable embodiment, the n-side cladding layer has a thickness of not less than 1.0 xcexcm and an average Al composition of not less than 5%. In a most preferable embodiment, the n-side cladding layer has a thickness of not less than 1.2 xcexcm and an average Al composition of not less than 4.5%.
In the sixth nitride semiconductor device, it is preferable that the p-side cladding layer is a super lattice layer comprising a third nitride semiconductor layer containing Al and has a thickness smaller than said n-side cladding layer. More preferably, the p-side cladding layer has a thickness of less than 1.0 xcexcm and the thickness of the n-side cladding layer and said p-side cladding layer including said active layer is set to range between 200 angstroms and 1.0 xcexcm.